Method for electrochemically covering an insulating substrate

ABSTRACT

The present invention relates to a process for obtaining a metal, ceramic or composite coating on the surface of a non-conductive material such as plastic, ceramic or wood which comprises: A) preparing a polypyrrole dispersion in aqueous base paint or in an acid type water-soluble pure resin; B) diluting the dispersion resulting from the previous stage with an alcohol in a factor of 1.5; C) applying the dispersion of the conductive polymer resulting from stage B) on the surface to be coated and drying same; and D) obtaining the metal, ceramic or composite coating by means of an electrolytic process or an electrophoretic deposition.

FIELD OF THE INVENTION

The present invention relates to obtaining a ceramic, metal or compositecoating on a non-conductive substrate, such as a plastic or ceramicsubstrate. More specifically the invention relates to a new process forobtaining a ceramic, metal or composite coating which comprisesgenerating, from a colloidal polypyrrole dispersion, a conductive layeron the non-conductive material and then carrying out an electrochemicalprocess.

BACKGROUND OF THE INVENTION

Various techniques for metallizing and depositing ceramic layers oninsulating materials such as plastic or ceramic are currently known inthe state of the art. The processes are generally based on theimpregnation of the surface of the insulating material with a materialallowing the passage of current in the processes for theelectrodeposition of metals or for the electrophoretic deposition ofceramic particles. In this sense, there are various technologies in thestate of the art for transforming an insulating surface into aconductive surface.

The metallization of a non-conductive material such as a ceramic or aplastic can be performed by means of different processes. The most usedconventional process comprises the following stages for generating aconductive layer on the surface of the insulating material: (i)mordanting or conditioning; (ii) neutralization (iii) activation orchanneling and (iv) chemical deposition. In the latter stage, a thinlayer of the metal, which in most cases does not reach one micron, isformed on the surface of the part, which becomes conductive.Occasionally, highly toxic Cr(VI) is used. This process generally hasthe serious drawback that very low thickness of this metal film areobtained, which makes it necessary to reinforce it by electrolyticallyapplying a second layer in order for it to have a suitable metal sectionfor the passage of the current in the points of contact with the frame.The second layer is obtained, for example, by applying a flash ofelectrolytic nickel and, thereafter, the successive metal layers ofcopper-nickel-chromium are applied.

Likewise, there are other alternative processes for metallizing plastic,among the method for etching non-conductive surfaces, such as ABS, witha solution of halogenides and/or nitrates described in EP 1785507 shouldbe mentioned. Other processes use solutions containing metal salts suchas silver, palladium, molybdenum for example. JP2005146330 thusdescribes a surface treatment method for a non-conductive material suchas ABS which uses the activation with Ag salts. JP62139896 describes theuse of polypyrrole in the process for metallizing a plastic, but it ismixed with the molten plastic itself. In another process, thenon-conductive surface is reacted with an alkaline permanganate solutionto form a manganese dioxide chemically fixed by adsorption; the surfaceis then contacted with an aqueous acid solution containing 0.1 to 200g/l of pyrrole or the derivative oligomers thereof, an electricallyconductive layer being formed; and finally an electric current suitablefor depositing the metal is applied. According to another process anoxidizer solution is applied to activate the non-conductive surface,followed by a treatment in a solution containing a heterocyclic monomersuch as pyrrole, thiophene, furan or the derivatives thereof forexample. The surface is then metallized by means of a galvanostatictechnique.

On the other hand, various processes for obtaining a metal coating on aceramic surface are likewise known. Thus, for example, WO 0104072describes a process which comprises impregnating a ceramic surface witha solution containing metal particles, such as a paint applied byspraying. The conductive surface obtained thus allows theelectrodeposition of a metal coating. U.S. Pat. No. 4,289,829 describesa (“moly-manganese”) process for metallizing a ceramic substrate whichcomprises applying a thin layer of paint containing molybdenum on theceramic surface and sintering the part under a reduced atmosphere. U.S.Pat. No. 3,998,602 describes a process for metallizing a non-conductivematerial which uses polymerizable monomers in the presence of smallamounts of silver ions.

The mentioned processes generally have various drawbacks. Thus, strongoxidizing substances such as Cr (VI), palladium salts, silver salts, aswell as the mixtures of substances which are highly toxic for theenvironment are used, and furthermore the number of stages of theprocesses is high, which lengthens the final process and makes it moreexpensive.

With respect to obtaining ceramic coatings on a non-conductivesubstrate, the existing techniques have experienced a great developmentsince the 1970s decade. In fact, there is currently a huge interest inobtaining ceramic coatings with improved and combined properties such ashardness, coefficient of friction, tenacity, electrical, optical,magnetic properties and corrosion resistance of the substrates forelectronic, medical and mechanical applications. Various processes whichuse the electrophoretic deposition and in which the substrate musttherefore be conductive are known in the state of the art.

In this sense, a process is known in which the coating is injected byapplying a current by means of a needle, also using small conductiveplates which are directly placed and spaced on the working area.

JP493970 describes a process for depositing ceramic particles whichconsists of formulating the conductive polymer in the presence ofceramic particles and thus forming a conductive layer thereon. WO200711010 describes obtaining metal and ceramic parts in a varied mannerby electrophoretic deposition. In this process, a metal or ceramicpowder suspension is injected into the substrate through a hollowneedle. The deposition of material occurs by applying a voltage betweenthe suspension and the substrate.

In any case, the state-of-the-art processes in which electrophoreticdeposition is used are always performed on the metal substrates and,furthermore, they later need a machining process.

Other state-of-the-art processes describe the manufacture of dentalprostheses of an exclusively ceramic nature, which are thereforebiocompatible, have a high resistance to masticatory forces and muchhigher esthetic values than those of ceramometal prostheses. Currently,in the manufacture of these ceramic dental prostheses, the applicationof the decorative ceramic layers giving the final finish to a ceramicprostheses (final enamel) is performed manually, by the application witha paintbrush, which slows down the process and makes it more expensive.Furthermore, the manual manufacture gives rise to the problems ofdimensional inaccuracy and lack of homogeneity in the volume of appliedmaterial, the subsequent correction and dimensional adjustment prior tothe implantation thereof being necessary.

Application US 2007029199 describes the formation of ceramic substratesfor dental applications by means of electrophoretic deposition. Metal orceramic substrates are used to which a layer of silver paint and ahygroscopic layer have been applied to prevent bubble formation. Thestate-of-the-art electrophoretic deposition processes are generallyapplied on metal substrates and may need an additional machining stage.

From what has been set forth above it is inferred that there is still aneed in the state of the art to provide an alternative process forobtaining ceramic, metal or composite coatings on a non-conductivesubstrate which overcomes at least part of the aforementioned drawbacks.

In this sense, the inventors of the present invention have discovered anew process by means of which it is possible to obtain a ceramic, metalor composite coating on an insulating substrate, based on the generationof a conductive layer from a colloidal polypyrrole dispersion on thesurface of the insulating substrate, and the use of said conductivelayer in obtaining one of said coatings electrochemically.

DESCRIPTION OF THE INVENTION

In one aspect, the invention relates to a process for obtaining aceramic, metal or composite coating on an insulating substrate whichcomprises the following stages:

A) preparing a polypyrrole dispersion in aqueous base paint or in anacid type water-soluble pure resin;

B) diluting the dispersion resulting from stage A) with an alcohol in afactor of 1.5;

C) applying the dispersion resulting from stage B) on the surface to becoated and drying same; and

D) obtaining the metal, ceramic or composite coating by means of anelectrolytic or electrophoretic deposition process.

The process, hereinafter the process of the invention, can be applied onany type of non-conductive substrate of a different nature, such as forexample plastic, ceramic, wood, among others.

The aqueous base paint useful to put the invention into practice can bea commercial outdoor acrylic water-based paint, and the acid typewater-soluble pure resin can be a acrylic resin in an aqueous base witha pH comprised between 2 and 5.

Stage A) of the process of the invention comprises obtaining in thefirst place a polypyrrole dispersion in aqueous medium by pyrrolepolymerization at room temperature. The resulting dispersion is taken todryness either by lyophilization or in a rotary evaporator for thecomplete evaporation of water. The dry material obtained is redispersedin a commercial aqueous base paint or in an acid type water-soluble pureresin, maintaining the stirring. In a particular embodiment, theconcentration of this dispersion is comprised between 0.05 and 2.5 gramof polypyrrole/g of paint or resin.

A particular embodiment of the process of the invention starts from themonomer pyrrole, at a concentration comprised between 1×10⁻⁴ and 1×10⁻¹g/ml in aqueous medium, and an anionic dispersant of the type ofpolystyrene sulfonate, poly(vinyl sulfonate), etc. with a molecularweight comprised between 10,000 and 1,000,000 g/mol. This mixture issubmerged in a bath at a temperature below 0° C. and stirred byultrasound. Once it is cooled, the oxidizer previously dissolved inwater, of the type of ammonium persulfate, iron trichloride, etc., isadded, following a process well known in the state of the art. Thepolymerization reaction is usually carried out at room temperature withmechanical stirring at 300 r.p.m, for a time usually comprised betweenhalf an hour and 7 hours. Once the reaction has ended, the stirring isstopped and the polypyrrole dispersion is removed from the bath.

In stage B) of the process of the invention, the polypyrrole dispersionin paint or resin is then diluted with an alcohol in a factor of 1.5until achieving a homogenous polypyrrole dispersion, ready to be used.In a particular embodiment, the alcohol is isopropanol.

The final steps of drying, redispersing and diluting are necessary forthe final paint or resin to not coagulate, be perfectly homogenous, havea lower viscosity, and not be as diluted as if the aqueous paint and thepolypyrrole dispersion were directly mixed without having beenpreviously taken to dryness.

Finally, the electrical conductivity of the conductive polymer can beoptionally determined by means of any conventional method.

Stage C) of the process of the invention refers to the application ofthe dispersion of the conductive polymer which is performed by theimmersion of the part to be coated, painting or spraying. In aparticular embodiment, the application is performed by the immersion ofthe part in the paint containing the colloidal polypyrrole andsubsequent drying in a temperature range comprised between 10 and 80°C., preferably room temperature. Finally, the part is washed withdistilled water.

Stage D) of the process consists of obtaining the final coating on thesurface to which the conductive polymer has been applied. The variousprocesses for obtaining each type of coating are described below:

D-1) Obtaining a metal coating by means of an electrolytic process: Itis necessary to have a simple cell containing the electrolyte and thecounter electrodes (anode and cathode) to obtain coatings of this type.Any conventional electrolyte or electrodeposition bath (silver, copper,nickel, gold, etc.) which is commonly used in conventionalindustrialized processes can be used to perform this galvanic process.The current used in the electrodeposition process can be direct currentor by means of current pulses (depending on the properties to beobtained in the final coating).

D-2) Obtaining a composite coating by means of a electrolytic process:It is necessary to have a simple cell containing the electrolyte and thecounter electrodes (anode and cathode) to obtain coatings of this type.Any conventional electrolyte or electrodeposition bath of those whichare commonly used in industrialized processes can be used as anelectrolyte. The electrolyte comprises at least one metal salt and atleast one ceramic particle. Examples of metal salts are salts of metalssuch as silver, copper, nickel, gold, among others. The ceramicparticles can be of SiC, Al₂O₃, SiO₂, ZrO₂, etc. The current used in theelectrodeposition process can be direct current or by means of currentpulses (depending on the properties to be obtained in the finalcoating).

D-3) Obtaining a ceramic coating by means of an electrophoreticdeposition process. This process is performed in the following steps:

Preparing the suspension of ceramic particles in an aqueous medium towhich the ceramic particles are added, at a concentration comprisedbetween 1-60% by weight, preferably between 4-30% by weight with respectto the total weight of the suspension. The types of ceramic particlesthat can be used in the present invention are, among others, oxides suchas SiO₂, ZrO₂, Al₂O₃, TiO₂; carbides, such as SiC, nitrides, such asNTi; and dental ceramic particles such as Base Dentine, Enamel, Dentine3M-2, Effect Peral, Effect Croma, which can be commercially acquired,for example, of the VITA Zahnfabrik trademark, as well as other ceramicswhich are conventionally used for generating esthetic layers on dentalceramic substrates.

A series of dispersant, binder and/or plasticizer additives are thenadded, always in a percentage which can vary between 0.1-2% by weightwith respect to the total weight of the suspension, for the purpose ofstabilizing the suspension. The additives used are: amines,polymethacrylates, pyrophosphates, citric acid, hydrochloric acid,methyl cellulose, etc. To homogenize and generate an electric chargedensity in the surface of the ceramic particles, they are stirred bymeans of ultrasound for a time ranging between 1 to 60 minutes.

The ceramic coating is obtained by means of an electrophoreticdeposition, for which a simple cell containing the previously preparedsuspension and the counter electrodes (anode and cathode) is used.Potentials varying between 20 and 400 V are applied for a time rangingbetween 1 and 40 minutes for the formation of the ceramic coating. Forthe consolidation of the coating, the latter is subjected to a sinteringprocess, applying temperatures comprised between 400° C. and 1500° C.,preferably at temperatures above 800° C.

The process of the present invention has a number of advantages incomparison with the processes of the art. It should be emphasized thatthe use of metal particles such as silver, of baths based on strongoxidizers such as Cr(VI), substances which are highly toxic for theenvironment, is eliminated. Furthermore, the number of stages necessaryfor generating a conductive layer on an insulating surface is reduced,with respect to conventional processes for metallizing plastic, to theapplication of the conductive polymer in an aqueous base and to thedrying thereof. Therefore, the process of the invention achieves, in asingle stage, transforming an insulating surface into a conductivesurface using a colloidal polypyrrole dispersion in an aqueous paint orresin, which furthermore considerably reduces the cost and time of theprocess.

On the other hand, the process of the invention has the additionaladvantage that it allows obtaining the final ceramic layers in a fewminutes, using to that end an electrophoretic deposition process. Thisprocess can be automated, further simplifying the process. The processof the invention therefore allows, in a particular embodiment, obtainingbiodegradable and ceramic dental prostheses quickly and with a highvolumetric accuracy. The process is performed on final ceramic teeth;the time of application of these decorative layers of ceramic is reducedfrom one day to a few minutes. The global process is thereforesimplified and the manufacturing time of the prostheses is significantlyreduced.

Illustrative examples of the invention are set forth below to betterunderstand the invention and in no case should they be considered as alimitation of the scope thereof.

EXAMPLE Example 1 Preparation of a Ceramic Coating for a DentalProsthesis

Aqueous polypyrrole dispersion was prepared in the first place. To thatend, a volume of pyrrole of 0.5-10 ml was added to an aqueous solutioncontaining 1-5% polystyrene in water. The resulting mixture wassubmerged in a bath at a temperature equal to 0° C. and stirred byultrasound. Once cooled, ammonium persulfate previously dissolved at aconcentration of 6.84 g in 50 ml of water was added. After two hours ofreaction, the stirring was stopped and the polypyrrole dispersion wasremoved from the bath. The resulting dispersion was lyophilized. Then,100 ml of commercial outdoor acrylic water-based paint (BRUGUER) weretaken, to which 15 g of lyophilized polypyrrole were added, maintainingthe stirring. To reduce the viscosity of the mixture, 25 ml ofisopropanol were used as a diluent. A homogenous polypyrrole dispersionin the ready paint was obtained, which was applied by means of theimmersion of a rectangular ABS (acrylonitrile-butadiene-styrenecopolymer) part obtained by injection.

An aqueous suspension of Enamel ceramic particles at a concentrationequal to 3% by weight was then prepared. The following additives werethen added: 10% polymethacrylate and 5% ethanol.

To homogenize and generate an electric charge density in the surface ofthe ceramic particles, they were stirred by means of ultrasound for 15minutes.

The part was then submerged in a simple cell containing the suspensionof ceramic particles. The part with the conductive polymer on itssurface was connected to one of the electrodes, and closing the electriccircuit, a metal counter electrode of stainless steel was used. Apotential of 100 V was applied for a time equal to 10 minutes for theformation of the ceramic coating.

For the consolidation of the coating, the part was subjected to asintering process which comprised subjecting the part to a pre-dryingtemperature of 500° C. for 6 minutes, applying a heating gradient of 55°C/min until reaching the cooking temperature of 910° C. and maintainingthis temperature for 1 minute.

A completely ceramic dental prosthesis was obtained.

Example 2 Preparation of a Metal Coating on Plastics

An aqueous polypyrrole dispersion was prepared in the first place as inExample 1. The dispersion was applied by means of immersion to the ABSpart.

The resulting part was subsequently introduced in a simple cellcontaining the counter electrodes (anode and cathode) and a conventionalelectrolyte of acid copper as an electrolyte or electrodeposition bath.The current used in the electrodeposition process was 3 A/dm².

An ABS part with homogenous and shiny metal appearance was obtained.

1. Process for obtaining a metal, ceramic or composite coating on thesurface of a non-conductive material which comprises the followingstages: A) preparing a polypyrrole dispersion in aqueous base paint orin an acid type water-soluble pure resin; B) diluting the dispersionresulting from the previous stage with an alcohol in a factor of 1.5; C)applying the dispersion resulting from stage B) on the surface to becoated and drying same; and D) obtaining the metal, ceramic or compositecoating by means of an electrolytic process or an electrophoreticdeposition.
 2. Process according to claim 1, wherein stage A) comprises:A-1) obtaining polypyrrole in aqueous medium from pyrrole; A-2) takingthe resulting dispersion to dryness; and A-3) redispersing the resultingdry material in aqueous base paint or in an acid type water-soluble pureresin until reaching a concentration comprised between 0.05 and 2.5 g ofpolypyrrole/g of paint or resin.
 3. Process according to claim 2,wherein stage A-1) is performed starting from pyrrole in aqueous mediumat a concentration comprised between 1×10⁻⁴ and 1×10⁻¹ g/ml; and ananionic dispersant with a molecular weight comprised between 10,000 and1,000,000 g/mol; the mixture is submerged in a bath at a temperaturebelow 0° C. and stirred by ultrasound; an oxidizer dissolved in water isadded; the polymerization takes place at room temperature withmechanical stirring, for a time comprised between half an hour and 7hours.
 4. Process according to claim 2, wherein stage A-2) compriseslyophilizing or treating in a rotary evaporator the polypyrroledispersion obtained in stage A-1).
 5. Process according to claim 2,wherein stage A-3) comprises dispersing the dry material obtained inA-2) in a aqueous base paint or in an acid type water-soluble pureresin, reaching a concentration comprised between 0.05 and 2.5 g ofpolypyrrole/g of paint or resin.
 6. Process according to the claim 1,wherein in stage B) the polypyrrole dispersion obtained in stage A) isthen diluted with an alcohol in a factor of 1.5.
 7. Process according toclaim 1, wherein stage C) of applying the dispersion of polypyrrole to apart is performed by the immersion of said part, painting or sprayingand subsequent drying.
 8. Process according to claim 1, wherein stage D)comprises obtaining a metal coating on the surface to which thepolypyrrole has been applied, by means of an electrolytic process in asimple cell containing an electrolyte with direct current or by means ofcurrent pulses.
 9. Process according to claim 1, wherein stage D)comprises obtaining a composite coating on the surface to which thepolypyrrole has been applied, by means of an electrolytic process in asimple cell containing an electrolyte which comprises at least one metalsalt and at least one ceramic particle.
 10. Process according to claim1, wherein stage D) comprises obtaining a ceramic coating on the surfaceto which the polypyrrole has been applied, by means of anelectrophoretic deposition in a simple cell containing a suspension ofceramic particles in an aqueous medium.
 11. Process according to claim10, wherein the concentration of ceramic particles in the suspension iscomprised between 1-60% by weight with respect to the total weight ofthe suspension and between 0.1-2% by weight with respect to total weightof the suspension of additives.
 12. Process according to claim 10,wherein a potential comprised between 20 and 400 V is applied for a timecomprising between 1 and 40 minutes.
 13. Process according to claim 10,wherein after the electrophoretic deposition the part obtained issintered at a temperature comprised between 400° C. and 1500° C.